The present invention relates to electric generators, and more specifically, to a spring mounting system supporting a stator core within a stator frame of such generators.
As the rotor of an electric generator rotates, its magnetic field revolves in unison with the rotor. This rotating magnetic field exerts a cyclical force on the generator's stator core. This force, in turn, causes vibrations to occur in the core which are of concern in high speed, high power machines. The stator core of a machine having a two-pole rotor experiences an elliptical deformation that follows the rotation of the rotor during operation, with resulting vibrations that are at a frequency twice that of the rotational speed of the rotor. The cyclical vibrations will be transmitted through the frame to the generator foundation. The magnitude will depend upon the degree of isolation incorporated in the design of the support system.
It is known in the art to utilize spring bars to resiliently support a stator core within a stator core frame. For example, U.S. Pat. No. 2,811,659, issued to Barlow, et al. on Oct. 29, 1957, incorporated by reference herein, discloses a stator core mounted in a stator frame by means of resilient support bars (spring bars) extending axially between ribs of a stator frame and the stator core. Barlow teaches that minimum contact between the spring bars and the stator core minimizes transmission of vibration.
Although the prior art has proven to be satisfactory, occasionally high elliptical mode stator frame vibration at two times rotational frequency is experienced. The amplitude of vibration will be determined by the proximity of the elliptical natural frequency of the stator frame to two times rotational frequency and to the magnitude of radial excitation exerted by the dilating stator core through the connecting spring bars.
An electric generator may be subjected to short circuit or other abnormal fault conditions during operation. High instantaneous torque loads can result depending upon the proximity of the torsional natural frequency of the spring mounted stator core to operating frequency. This large torque amplification is imposed upon the spring bars and the connections to the stator frame through to the foundation and can mandate robust support structure and foundation bolting requirements. The torque load amplification during short circuit conditions is a function of the air gap torque, the system torsional natural frequency, and the rotor rotating frequency. The air gap torque and the rotor frequency are functions of the generator design. However, by controlling the system natural frequency, the off-normal torque loading imposed on the generator support system can be minimized. From the literature it is known that vibration isolation can be achieved when the ratio of forcing frequency to natural frequency is .sqroot.2 or preferably greater if significant reduction in response is desired. Therefore, it is desirable to "low tune" the torsional natural frequency of the support system. Although the prior art is based upon this recognition, the reduction in torsional natural frequency of the spring mounted stator core is limited by the resulting high stress and consequent material strength requirements as the spring bars of the prior art are slenderized.
The object of the present invention, therefore, is to provide a support structure that is capable of reducing the torque amplification under short circuit conditions, slenderizing and reducing the stresses in the spring bars during short circuit conditions, and reducing the magnitude of radial excitation force transmitted by the spring bars during normal operation.